Charge Storage Mechanism of Drain Induced Dynamic Threshold Voltage Shift in p -GaN Gate HEMTs

The drain induced dynamic threshold voltage (V ₓ₇) shift of a p -GaN gate HEMT with a Schottky gate contact is investigated, and the underlying mechanisms are explained with a charge storage model. When the device experiences a high drain bias V ₃ₒₐ, the gate-to-drain capacitance (C ₆₃) is charged to Q ₆₃ (V ₃ₒₐ). As the drain voltage drops to V ₃ₒ₌ where V ₓ₇ is measured, C ₆₃ is expected to be discharged to Q ₆₃ (V ₃ₒ₌). However, the metal/ p -GaN Schottky junction could block the discharging current, resulting in storage of negative charges in the p -GaN layer. For the device to turn on, additional gate voltage is required to counteract the stored negative charges, resulting in a positive shift of V ₓ₇. The dynamic V ₓ₇ shift is an intrinsic and predictable characteristic of the p -GaN gate HEMT which is linearly correlated with \!Q ₆₃=Q ₆₃ (V ₃ₒₐ) - Q ₆₃ (V ₃ₒ₌). The V ₓ₇ shift is dependent on V ₃ₒₐ as well as V ₃ₒ₌, indicating that the V ₓ₇ shift is varying along the load line during a switching operation.